In commonly owned U.S. Pat. No. 5,630,796, a noninvasive delivery system is described that entails the use of a needleless syringe device. The syringe is used for transdermal delivery of powdered therapeutic compounds and compositions to skin, muscle, blood or lymph. The syringe can also be used in conjunction with surgery to deliver therapeutics to organ surfaces, solid tumors and/or to surgical cavities (e.g., tumor beds or cavities after tumor resection).
The needleless syringe is constructed as an elongate tubular nozzle. Particles of a powdered therapeutic agent are located adjacent to the upstream end of the nozzle. The mechanics at the upstream end of the nozzle may involve a reservoir of gas such as helium, at a high pressure (e.g., between about 30 and 80 bar), and a high pressure gas flow is created by releasing the gas suddenly from the reservoir (e.g., by opening a valve), or by releasing the gas from the reservoir so that pressure builds up behind a membrane at the entrance to the nozzle until the membrane eventually ruptures to release the high speed gas flow. The particles may be contained initially within the high pressure gas reservoir or within a cassette from which the particles are released into the gas flow. The passageway through the nozzle itself is preferably convergent/divergent or convergent/cylindrical, with the upstream convergent section shorter than the downstream divergent or cylindrical section.
Since the gas pressure release from the above-described needleless syringe device is generally at or approaching supersonic speeds and can further involve the rupture of one or several membranes, the actual operation of the device can be quite noisy. In order to address this issue, the device was provided with a tubular shroud having a cylindrical silencer portion surrounding a downstream section of the nozzle. A spacer portion of the shroud extends axially beyond the exit plane of the nozzle to abut the target surface and positively space the exit plane of the nozzle from the target surface. An annular chamber was created between the nozzle and cylindrical silencer portion, wherein the chamber includes a series of baffles to create a tortuous path for escaping gas to pass through on its way through an upstream end of the silencer portion where several ports were provided for venting the gas to the atmosphere. Such a shroud is illustrated in FIGS. 1 to 3 of U.S. Pat. No. 5,630,796. Although practical for its intended purpose, such a shroud is still noisy owing to the comparatively large vent ports. In addition, it has been found that the tubular shroud configuration allows for particles reflected from the target surface to pass out of the ports even after multiple reflections from the series of baffles. Accordingly, there remains a need to provide a silencer mechanism that reduces the noise associated with operation of the needleless syringe and which contains any particles which may reflect from the target surface.